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What Is Shot Blasting?
This process is a highly effective solution for removing contamination on metal substrates or changing the coarseness or smoothness of a surface before coating. The force in which the abrasive is propelled can be generated either centrifugally, using a wheel spinning at a high RPM, or pneumatically through the medium of compressed fluid or air.
Technically, shot blasting refers specifically to a process that uses spherical particles. Not to be confused with grit blasting which uses angular or sub-angular grains.
What does the term “blast media” mean, and which abrasive is right for me?
“Media” is the common term used when describing the abrasive, which can come in many variations. These characteristics determine abrasive type; the grade (the size of the media), hardness (measured in Mohs), shape (spherical or angular), and mineral composition. Each of these factors affects the finished surface profile. For assistance when selecting media that’s right for you, please contact our sales team.
Grit blasting is sometimes used as a general term for the process of cleaning or providing a profile on a hard surface. “Grit” blasting specifically means blasting with angular or sub-angular particles. The effect of using grit abrasive is like that of using sandpaper, although grit blasting provides a more even finish and is ultimately a much faster process.
Wet blasting is favoured for maritime and outdoor applications due to the cushioning effect of the water, which reduces the amount of dust created. However, wet blasting has a much slower cleaning action than air-only systems and can cause “flash rusting” on certain ferrous surfaces.
Bead Blasting or Shot Peening
Bead blasting, also known as “shot peening”, is a process of using circular media to dimple the surface of an object. Shot peening is like grit blasting but works on the mechanism of plasticity rather than abrasion, which removes less material and generates less dust. Additionally, shot peening is used to relieve the tensile stress of metal and composites, increasing its compressive strength. This process is usually reserved for delicate machine components such as gear parts, suspension springs, and turbine blades.
Wheel blasting uses centrifugal force to propel media against a surface. A spinning wheel rotates at exceptional speeds to throw abrasive in a self-contained cage. Wheel blasting is often described as an airless blasting operation because of the lack of compressive propellant. It is a highly efficient form of processing that makes use of recyclable media for prolonged operation. However, this form of blasting lends itself to repeated shapes/products and is not as versatile as a blast room.
Mobile blast systems, or “blast pots” are compressor-fed, pneumatic machines, filled with abrasive material. Abrasive is held inside a pressurised vessel and is ejected out of a hose, manipulated by the operator. The operator can control the flow of media through the use of a deadman handle.
Blast cabinets are small closed-loop systems designed for smaller component processing. Cabinets exclusively use recyclable abrasive and have four individual modules; the containment (cabinet), the abrasive recycling system, the blasting abrasive, and the dust collector. Operators are able to manipulate components and the blast hose inside, through the use of rubber gloves integrated into the cabinet wall. A viewing window above the gloves allows the operator to see what they are working on. Blast cabinets can either work with a pressurised pot or a suction system which inducts media into an air stream and blasts at lower pressures.
Blast rooms are a much larger version of a blast cabinet used for heavy industrial applications. Operators need to be trained to work in these rooms. Inside, the operator roughens, smooths, or cleans surfaces of a component, depending on the needs of the finished product. Blast rooms come in a range of sizes. As a result, they can accommodate very large or uniquely shaped objects, such as rail cars, commercial or military vehicles, construction equipment, ship sections, and aircraft. Blast rooms (and cabinets) can also be automated. This is achieved by installing a conveyance system and robotic nozzles inside.
Blast rooms have several key components:
- The enclosure: the sealed space where operators work inside.
- The pneumatic blast unit, or several units.
- A dust collector, which extracts particulate matter out of the blast room’s atmosphere.
- The abrasive media reclamation system: which recovers ejected abrasive, filters it and delivers it back to the blast machine for reuse.
There are a large number of media types to consider when grit blasting and each has a different effect on the substrate they are used on. As you gain experience in the field of shot blasting, you will get to know the media types and the tasks they are commonly associated with. For example, some cannot be used on certain surfaces i.e., ferrous media on a nonferrous substrate. Certain media types are more aggressive than others. A number are favoured because they provide specific cosmetic finishes. Some are intended to relieve tensile stress, and others are suited to being reclaimed and used again.
Location Also Affects Media Choice
Before blasting, it is vital to consider the material you’re working with and where you’re blasting. Both factors will determine the media you can use to complete the job without damaging the component in the process. For instance, removing heavy corrosion on large steel ferrous sheets, a harder, reclaimable, more aggressive aggregate such as chilled iron would be an appropriate choice for a blast booth.
However, when using a portable blast machine blasting outdoors, copper slag would be a better choice as it is an expendable abrasive and is more environmentally friendly.